In recent years, ink-jet recording systems have been applied to various printing fields such as photography, various forms of printing, marking, and special printing such as color filters due to its capability of forming images simply and inexpensively. Specifically, it has become possible to achieve image quality comparable to that of silver salt photography, by employing recording apparatuses which eject minute dots under control, inks which exhibit an enhanced color reproduction range as well as enhanced durability, and improved ejection adaptability, and exclusive sheets which exhibit markedly improved ink absorbability, color forming properties of colorants and surface glossiness. Enhancement of image quality of the current ink-jet recording system is achievable only by simultaneous improvement of all the recording apparatuses, inks and exclusive sheets.
However, ink-jet systems, which require special recording sheets, result in problems such as limitations of recording media as well as a higher cost of the foregoing recording media. As a result, a number of experiments have been conducted in which recording is carried out on image receiving media differing from such exclusive sheets, while employing various ink-jet systems. Specific examples include a phase change ink-jet system using a wax based ink which is solid at room temperature, a solvent based ink-jet system using an ink comprised of rapid drying organic solvents as a main component, and a UV ink-jet system in which after image recording, crosslinking is carried out employing ultraviolet (UV light) radiation.
Of these, in recent years, the UV ink-jet system has received much attention due to its relatively low generation of unpleasant odors, rapid drying properties, and recording capability onto recording media exhibiting no ink absorbability. Ultraviolet radiation curing ink-jet inks are disclosed, for example, in Japanese Patent Publication No. 5-054667, Unexamined Japanese Patent Application Publication 6-200204, and Unexamined Japanese Patent Application Publication (under PCT Application) No. 2000-504778.
However, even though these inks are employed, it is very difficult to form high-definition images on each of the recording materials due to large differences of ink dot diameter after the ink deposition, depending on the type of recording materials and operation environment.
Specifically, UV radiation curable inks using a cationic polymerizable compound have been proposed (for, example, please refer to Patent Documents 1-4) in recent years, however, these UV radiation curable inks have drawbacks which tend to be affected by molecular level moisture (humidity), even exhibiting no oxygen inhibition action. Further, these inks have drawbacks of wrinkling generation by shrinkage during curing depending on curing conditions.
Image forming with a UV radiation curable ink jet method is characterized by having a capability of printing onto recording media exhibiting no ink absorbability. However, there are the specific drawbacks for this UV radiation curable ink jet method. For example, in the case of image formation onto a non-ink-absorbable recording medium, image quality deterioration due to mixture of deposited dots with neighboring dots is remarkable. Specifically, when a high quality image is needed, dot mixture among deposited different colors results in a big problem. Regarding above problems, proposed are active ray curable inks employing an oxirane compound, a vinyl ether compound or an oxcetane compound as a photopolymerizable compound (for example, please refer Patent Documents 1 and 2). Further, also proposed are active ray curable inks employing alicyclic epoxy compounds and oxetane compounds as photopolymerizable compounds (for example, please refer to Patent Documents 3 and 4). However, with the methods proposed above, dot bleeding is not satisfactorily solved yet in the present situation.
In the case of the commonly used water based ink jet method, these problems have been solved by enhancing ink permeability into a recording medium using an exclusive sheet or additives having each of functions in inks. Under the present situation, in a UV radiation curable ink jet method, there is no solution for image formation onto a non-ink-absorbable recording medium. Further, in high speed printing, the materials printed an image thereon are usually wound up in to a roll. Therefore, development of an active ray curable ink-jet ink composition is specifically desired.                Patent Document 1: Unexamined Japanese Patent Publication (hereinafter, referred to as JP-A) 2001-220526 (in claims and examples)        Patent Document 2: JP-A 2002-188025 (in claims and examples)        Patent Document 3: JP-A 2002-317139 (in claims and examples)        Patent Document 4: JP-A 2003-55449 (in claims and examples).        